An electric vehicle drives an electric motor in accordance with a range selected by a shift manipulation which is done by a driver to produce a running mode of the vehicle. By transmitting this power from the electric motor to road-wheels, an electric running can be realized.
When the electric vehicle is made to run at very low speed (is made to creep) by creep torque with a forward-or-reverse-running range selected as in the case of automatic-transmission vehicle; the electric motor is driven and controlled to output the minute creep torque from the electric motor, and this creep torque is transmitted to the road-wheels to enable the creep running of the electric vehicle.
For example, Patent Literature 1 proposes a creep-running control technique for an electric vehicle. In this technique, in addition to enabling the creep running of the electric vehicle as mentioned above, a creep cutoff which reduces the creep torque of the electric motor (down to zero) is performed when a predetermined creep-cutoff permitting condition is satisfied. This predetermined condition is that a detection value of vehicle speed is lower than a set value and also a braking force is larger than or equal to a set value, for example, when the vehicle is in a stopped state with no intention to start moving the vehicle. Such a creep cutoff is performed because the creep running is not conducted immediately and also because a power consumption can be suppressed.
However, in the creep-cutoff control device for electric vehicle as disclosed in Patent Literature 1, the creep cutoff which reduces the creep torque of the electric motor (down to zero) is unconditionally performed when the predetermined creep-cutoff permitting condition is satisfied, for example, when the vehicle is in a stopped state with no intention to start moving the vehicle. Therefore, the following problem is caused.
For example, a case will now be discussed where a driver is adjusting the vehicle speed around 0 by a delicate increase/decrease of braking force through a brake-pedal manipulation while the electric vehicle is outputting the creep torque in a forward direction because of the setting of forward-running range (DRIVE-mode).
In this case, the creep-cutoff permitting condition related to the vehicle speed has been satisfied because the vehicle speed is around 0. On the other hand, the creep-cutoff permitting condition related to the braking force is not satisfied because the braking force is being used to adjust the vehicle speed around 0 in relation to the minute creep torque and hence is not larger than or equal to the above-mentioned set value. Therefore, in this situation, the creep torque is being outputted from the electric motor without executing the creep cutoff.
If the driver increases the braking force carelessly or in order to stop the vehicle, the braking force becomes larger than or equal to the set value so that the creep-cutoff permitting condition related to the braking force is satisfied. At this time, both of the creep-cutoff permitting condition related to the vehicle speed and the creep-cutoff permitting condition related to the braking force are satisfied, so that the creep torque is rapidly brought to 0 by the execution of the creep cutoff.
Before the creep torque is brought to 0 by such a creep cutoff, the driver has focused his attention on adjusting the vehicle speed around 0 by the brake manipulation as mentioned above and hence is not aware of changing the torque at all. Therefore, when the creep torque is brought to 0 by the creep cutoff, the driver feels a torque reduction unrelated to his own driving manipulations, i.e., an unintended torque reduction. Hence, a problem is caused that the driver has a feeling of strangeness.
This problem is caused similarly also in a case that the driver is adjusting the speed of the electric vehicle around 0 by a delicate brake manipulation while the electric vehicle is outputting the creep torque in a reverse direction because of the setting of reverse-running range (REVERSE-mode).